1. Field of the Invention
This invention relates to an exposure device, an LED print head, and an image forming apparatus having the exposure device and the LED print head.
2. Description of Related Art
Conventionally, to form an exposure image with an image forming apparatus such as, e.g., a printer, a facsimile machine, a photocopier, or the like, of an electrophotographic type, during an exposure process for forming an electrostatic latent image on a photosensitive drum based on an image data, an LED (Light Emitting Diode) array of arrayed LED elements in a plurality number is used as a light source and an optical image from a light emitting point of the LED array is focused on the photosensitive drum in using a lens. A lens array is used as the lens in many cases, in which a plurality of lens elements are arranged in parallel rows (see, e.g., Japanese Patent Application Publication No. JA-H7-242,018).
In the meantime, a lens array is used in the exposure device, in which a rod lens having a refraction index distribution in a direction perpendicular to an optical axis of the lens element, i.e., in a radial direction with respect to a center of the lens, is used as the lens element.
To structure the lens array, lens elements in a cylindrical shape, having an external diameter of 0.1 to a few millimeters, are arranged in one or multiple lines in a linear manner, thereby being securely immobilized with, e.g., a resin or the like, serving as a filling material for filling up gaps among the lens elements.
On the exposure device with use of the lens array, luminosity at the time of exposure on a surface of the photosensitive drum depends on an aperture angle of the lens element composed of the rod lens, and the luminosity becomes higher as the aperture angle becomes larger. The aperture angle of the lens element in current use is set to between ten and thirty degrees, and the aperture angle of the lens element composing the lens array is desirably set to as large as possible especially in a case of the image forming apparatus in using, as the light source, the LED array with use of the LED elements having lower light intensity compared with, e.g., a semiconductor laser or the like. The aperture angle of the lens element composed of the rod lens composing the lens array currently used for the exposure device of the image forming apparatus is set to approximately twenty degrees.
Where using the lens element having the large aperture angle, however, a comatic aberration causes, which is resulted from the high refractive index at a position distant from a center of the lens element. That is, a distance between a position at which a light transmitted through a position distance from the optical axis of the lens element having the large aperture angle is focused into and a lens surface, is shorter compared with a focal distance, i.e., a distance between a position at which a light transmitted through the vicinity of a center in the lens element is focused into an image and a lens surface.
Therefore, with the lens array with use of the lens elements having the large aperture angle, a focused image defocuses due to the comatic aberration, thereby causing such a defocusing that the exposure image of the light emitting point of the LED element serving as the light source is not focused on the photosensitive drum, so that the light intensity becomes uneven at each point of the exposure image or a contrast deteriorates at the exposure images. Furthermore, on a printing with use of the image forming apparatus, the defocusing due to the comatic aberration causes irregularity in concentration or deterioration in a dot reproducibility such as resulting in unclear printed images.
The defocusing due to the comatic aberration is described next.
FIG. 2 is a view illustrating the defocusing due to the conventional comatic aberration.
FIG. 2 shows a relation between the lens element composed of the rod lens and an image-focused surface on which the light transmitted through the lens element is focused. A light source point, not shown, is to be positioned at the opposite side across the lens element from the image-focused surface. Herein, marks O1, O2, and O3 express respectively the lights transmitted from the light source point, in which the mark O2 expresses the light transmitted through a portion more distant from an optical axis of the lens element compared with the mark O1 while the mark O3 expresses the light transmitted through a portion even more distant from the optical axis of the lens element. On the image-focused surface, points on which these lights are focused are respectively set to marks P1, P2, and P3. Shifts among these points on which the lights are focused is defined as the comatic aberration as well as a cause for the defocusing of the exposure image on the image-focused surface. On the condition that distances respectively between these points on the image-focused surface, on which the lights are focused, and a central point of the lens on the lens surface are respectively set to f1, f2, and f3, inequality, f1<f2<f3, is set.
Herein, with the lens array in using the lens elements having the large aperture angle, to improve the deterioration of the contrast at the exposure image due to the defocusing of the focused image resulted from the comatic aberration, it has been carried out to arrange a diaphragm on the exposure device of the image forming apparatus, in which the diaphragm shields against the light incident on or exiting from the portion distant from the central point of each lens element composing the lens array. That is, a depth of field becomes deeper to solve the defocusing, upon shielding against the lights O2, O3 transmitted through the position comparatively distant from the center of the lens element in FIG. 2.
Furthermore, not only the solution of the defective image and the improvement in image quality, but also further improvement in resolution is expected based on the current situation that the resolution has been improved lately on the general image forming apparatus including that of electrophotographic type. As a method for solving deterioration in the exposure image with the lens array, it has been conventionally considered as effective to arrange a shielding member to the lens array.
However, on the lens array used for the exposure device, such as described above, deviation occurs undesirably in each of the arrayed lens elements composed of the rod lens. In this case, the deviation in the arrayed lens elements is resulted from, for example, misalignment in a direction of the array of the lens elements or in a direction perpendicular to the direction of the array of the lens elements as well as in a direction perpendicular to a direction of the optical axis of each lens element, or resulted from slant due to tilting with respect to the original direction of the optical axis.
For example, the misalignment of each lens elements in the lens array used for the exposure device of the electrophotographic image forming apparatus, in the array direction, is approximately within a few percent where indicated with a value with respect to a design value. Furthermore, the extent of deviation in this misalignment is different depending on each position of the lens array, such as, e.g., an end portion, a central portion, or the like.
Herein, one example of a structure and a fabrication method of the lens array are described in reference to a drawing.
FIG. 3 is a partially cutout perspective view showing the conventional lens array.
To structure the lens array, lens elements 201 composed of the rod lens having the refractive index distribution are arranged in one or multiple rows to one backing plate 202, and subsequently synthetic resins serving as a filling material 203 is poured into gaps among the lens element 201, and further the lens elements 201 are securely immobilized upon pressed with the other backing plate 202. At that time, in a case of the backing plate 202 in a flat plate form, an array pitch of the lens elements 201 is equal to a diameter of the lens element 201, while in a case where concaves and convexes with prescribed intervals are formed onto the backing plate 202 serving as a frame plate, a pitch between the concave and the convex is set to the array pitch of the lens element 201.
These misalignment in the array or the slant of the lens element 201 is presumably caused by, e.g., deviation in the diameter of the lens element 201, twisting or bending of the lens element 201, bending or winding of the backing plate 202, deviation in the position or the shape of the concave and the convex formed on the backing plate 202, for determining the interval between the arrayed lens elements 201, deviation in fabrication at the time of securely immobilizing the lens elements 201, deviation in characteristic such as, e.g., viscosity etc., of the synthetic resin to be filled, deviation in the amount of the filled synthetic resins, or the like.
Furthermore, deviation occurs in the shape, such as due to twisting, bending, or the like, on the entire lens array.
With a molding process, an art widely known in general as a method for manufacturing the shielding member, where openings in a large number are formed to a width of the paper corresponding to a width of the image forming apparatus, with a positional accuracy of approximately 0.1 to a few millimeters corresponding to the array of the lens elements in the lens array, the opening of the shielding member has a positional accuracy of approximately several dozen percent, and even in a case with an etching process with use of a photomask or a pattern formation of a shielding pattern, the positional accuracy is set to approximately a few percent. Even where the openings are formed with a cutting process or a punching process, the positional accuracy is set to approximately several dozen percent. Furthermore, with these processing methods, since accumulated error occurs even where the positional accuracy of each opening of the shielding member is limited to a few percent, each opening undesirably gets out of position with respect to the primary intervals among the arrayed lens elements, determined with the design value.
However, with the conventional exposure device described above, where a center position of the opening on the shielding member is displaced relative to a center position of each lens element composed of the rod lens in the lens array, the light from not only a peripheral portion but also the vicinity of a center of the each lens element are cut off, so that such an effect undesirably reduces by half, as resulted from setting of the shielding member for improving an MTF (Modulation Transfer Function) of the exposure device. Furthermore, where the center position of the opening on the shielding member is displaced widely relative to the center position of each lens element in the lens array, the vicinity of the center of the lens element is shielded periodically to decrease the exposure amount in large, thereby causing problems such that white streaks appear periodically, or that the exposure image is not formed at all on the photosensitive drum so the image formation itself as to become unimplementable.
Furthermore, since the slant lens element leads to deterioration in the MTF of the exposure image with the lens element near the slant lens element, the exposure image becomes unclear, thereby causing the defective image such as having vertical streaks or the like, on the printed image.
With the exposure device generally used for the electrophotgraphic image forming apparatus, the defective image such as having the vertical streaks or the like due to deviation in the light intensity among the plural light sources, is to be improved upon correction of the light intensity of each light emitting point in the LED array for forming the exposure image. However, since the defective image such as having the vertical streaks or the like due to the slant lens elements is resulted from deterioration in the MTF of the exposure image near the slant lens element, such a defective image cannot be improved only upon the correction of the light intensity of each light emitting point in the LED array.
The deterioration in the MTF due to the slant lens elements composed of the rod lens will be described next.
FIG. 4 is a view showing a typical relation between the light emitting point and distribution of the light intensity at the exposure image with each lens element in the conventional lens array.
Herein, the light intensity at the time that two light emitting points are arranged as a light emitting point, with a minute interval, is considered. With the exposure device with use of the lens array, the exposure image from the light emitting point results in the exposure images respectively overlapped with lens elements composed of the rod lens or in the exposure image formed on the image-focused surface. On placing the light emitting points and the lens as shown in FIG. 4 (a), on the basis that the origin is set to a position of the exposure image from the light emitting point in a case where the lens elements in the lens array are not slanted, a direction parallel to the optical axis of each lens element, to which the light emitting point extend from the origin, is set to alphabet z, while the array direction of the lens elements is set to alphabet x. Where there exists a defectively slant lens element, such as shown in FIG. 4 (a), a position at which the exposure image from the light emitting point is formed with the defectively slant lens element is displaced relative to the primary place at which the exposure image is to be formed. Therefore, the distribution of the light intensity near the origin of the exposure image from the light emitting point in a case with the defectively slant lens element, is such as shown in FIG. 4 (d) and FIG. 4 (e), in which the light intensity decreases while the MTF of the exposure image deteriorates. Furthermore, as shown in FIG. 4 (f), since added with the exposure image from the light emitting point with the defectively slant lens element, the distribution of the light intensity changes at the exposure image as actually formed, defined as the exposure images overlapped respectively with the lens elements, compared with the primary distribution of the light intensity of the light emitting point, and therefore the MTF deteriorates.
This invention is aimed to solve the aforementioned conventional problems of the conventional exposure device and to provide an exposure device, an LED print head, and an image forming apparatus having the exposure device and the LED print head, in which the shape or position of a shielding member is set corresponding to an image focusing property of each lens element so as to prevent an MTF from deteriorating, to provide a clear exposure image, and to prevent a defective image such as having a vertical streak or the like, from occurring on a printed image.